1. Field of the Invention
The present invention relates to a charging method and a charging device for charging a charge receiving member. In particular, the invention relates to a charging method and a charging device for charging a charge receiving member through a portion in contact with the charge receiving member. More specifically, the invention relates to a charging method and a charging device having a member to be in contact with an electrostatic latent image carrier such as a photosensitive member in an image forming apparatus for charging the electrostatic latent image carrier to a predetermined potential.
2. Description of the Related Art
In electrophotographic image forming apparatuses such as a copying machine and a printer, a charging device charges an electrostatic latent image carrier such as a photosensitive drum, image exposure is effected on a charged area to form an electrostatic latent image, the electrostatic latent image is developed into a visible image, and this visible image is transferred and fixed onto a transfer member. A charging device is employed for charging the surface of the electrostatic latent image carrier prior to formation of the latent image. The apparatus also employs other charging devices such as a transfer charger for transferring the visible image formed on the electrostatic latent image carrier onto a transfer member and a separating charger for separating the transfer member bearing the transferred visible image from the electrostatic latent image carrier.
As the charging devices, corona charging devices such as a Corotron charging device or a Scorotron charging device utilizing corona discharging have been used. Although the corona charging device can advantageously perform stable charging, it requires a high voltage and produces a large amount of ozone. In recent years, therefore, contact charging devices which can replace the corona charging devices have been proposed.
The contact charging device has a charging member which is subjected to a voltage and is brought into contact with a charge receiving member. The charge receiving member is charged by discharge occurring at a gap between a region, which is continuous to a contact region of the charging member with the charge receiving member and is spaced from the charge receiving member, and the surface of the charge receiving member.
The charging member may take various forms such as a charging roller and a charging brush, and other forms such as a charging blade and a charging film have been proposed.
These contact charging devices have such an advantage that they produce a particularly smaller amount of ozone than the corona charging device. Also, the contact charging device can operate with a voltage lower than that required by the corona charging device.
However, the contact charging device may suffer from the following problem. If the charging member is made of a material of a low resistance such as metal, the surface of the charge receiving member is irregularly charged into a scale-like form (scaly form). Therefore, if the charge receiving member is an electrostatic latent image carrier and a mesh image of 1-dot/4-dots is formed as shown in FIG. 4, irregularity in image density occurs in the mesh image due to lack and/or drop of dots forming mesh points. This results in scale-like noises as shown in FIG. 9. This means that the surface of the charge receiving member was not uniformly charged, and charging irregularity occurred in a scale-like form. This problem does not occur if the charging member is made of a material of a high resistance. Therefore, it has been impossible to employ the charging member of a low resistance such as metal, and it has been required to employ the charging member made of a material of a high resistance such as rubber or resin containing an electrically conductive material dispersed therein.
Charging irregularity in a striped form may occur at the surface of the charge receiving member when a scratch is formed at a portion of the charging member in contact with the charge receiving member and the scratch is stuffed with a foreign material such as toner. For suppressing this, the charging member may be made of a hard material. Such a hard material may be metal. However, metal cannot be employed due to the aforementioned reason. Therefore, it is necessary to employ a charging member made of a material which is formed of specific hard resin and a conductive material such as electrically conductive carbon dispersed therein. The same is true with respect to the case where a charging member having a sufficient resistance against wear is employed.
For the reasons described above, a material of the charging member can be selected only from a limited range.